Global Navigation Satellite System (GNSS) is a general term for all modern satellite-supported positioning systems. Currently in operation are the GLONASS, which is operated by the Russian Ministry of Defence, and the Global Positioning System (GPS), which is operated by the US Department of Defence. The Galileo system is also being established. The invention described below can be used in any of these satellite-supported positioning systems. The first system to be developed was the GPS, which is currently the most frequently used one. The system comprises a nominal configuration of 24 satellites with sufficient redundancy so that typically visual contact with about six satellites exists at any time and from any location. For determining the three-dimensional position of a user, it is necessary to receive at least four satellite signals. The accuracy of positioning is limited to about 10 m.
A relative GNSS is a technique for increasing the achievable accuracy of positioning to a submeter range. For this purpose, a GNSS receiver is set up at a location having a known three-dimensional position, the GNSS receiver thus acting as a reference station. By comparison of the position determined by means of the GNSS signals with the known position, influences having an adverse affect on the accuracy of positioning, such as, for example, refraction or orbital errors, can be substantially reduced.
The calculated correction data can now be sent by means of an additional radio connection to one or more user stations in order to permit correspondingly accurate positioning of the user stations.
There are two possibilities for relative GNSS positioning: differential GNSS positioning (DGNSS) and real time kinematic positioning (RTK). In the case of RTK positioning, a higher data flow of correction data is necessary, it also being possible to achieve a higher accuracy of positioning. Thus, in DGNSS positioning, an accuracy in the meter range is possible, whereas an accuracy in the centimetre range can be achieved in RTK positioning.
An example of relative GNSS positioning for surveying work is shown in FIG. 1. A GNSS receiver 103 of a reference station 101 receives position signals 104 from satellites via a GNSS antenna 102. In addition, the exact fixed position of the reference station 101 is known. The position signals 104 are processed to give correction data 105. Correction data or correction information 105 means in this context positions, status information, satellite measurement data and/or measurement correction data, etc. Such correction data can be provided, for example, in standardized data formats, such as CMR, CMR+, RTCM 2.x, RTCM 3.x. The correction data 105 are transmitted by means of a radio device 106 in a certain frequency band via the radio antenna 107.
A user station 111 receives the GNSS correction data 105 by means of a radio device 116 via the radio antenna 117 and passes on said correction data to the GNSS receiver 113 of the user station 111. At the same time, the GNSS receiver 113 of the user station receives GNSS position signals 114 via a GNSS antenna 112. By means of the correction data 105, a correction of the position of the user station 111 determined by the GNSS position signals, is possible.
Modern GNSS stations can act both as a user station and, in the case of known positioning, as a reference station.
Since a standardized data format is used for the GNSS correction data, it is possible to combine equipment from different suppliers. However, the radio frequencies at which the waves carrying the GNSS correction data are transmitted and received are not standardized.
In choosing suitable radio frequencies or communication services for the GNSS data transmission, technical, economic and administrative aspects play a role.
In general, the following is true: the lower the frequency, the greater is the possible distance between the transmitter of the correction data and the receiver. At the same time, however, the following is also true: the higher the frequency, the higher is the possible data transmission rate.
Radio frequencies are, however, not freely useable. The use is regulated by international agreements and national laws.
For example, frequencies in the lower microwave range, on which it is possible to operate without permission at low power, are available in large parts of Europe. The range of the transmitter is limited to a few kilometres.
GNSS corrections are broadcast over an extensive area in many countries as part of the data transmission operated by broadcasters with the aid of the Radio Data System (RDS).
Mobile telephone frequencies and internet services are also frequently used for the transmission of correction information.
In order to permit a high degree of combinability with different systems by coverage of different frequencies for correction data transmission in the case of GNSS receiver stations, known GNSS stations 101, as shown in FIG. 1, are formed, for example, with a holder for a radio module 106, the radio module 106 containing both a radio modem and a downward-pointing rod antenna 107 and being connected by the plug connection to the GNSS receiver 103. Depending on the frequency used in the system in which the GNSS station 101 is employed, the corresponding radio module 106 can be employed in the GNSS station 101 for communication of GNSS correction data 105. The radio module 106 then receives correction data 105 which are transmitted to the GNSS receiver 103 and are used for correction of the measured position. However, in the case of a rod antenna 107 arranged next to the surveyor's staff 108, the fact that it may be destroyed on the ground by environmental influences and/or mechanical damage, does not have an optimal radiation characteristic—in particular owing to signal wave obscurations by the plumbing staff or by a user holding the plumbing staff 108—and covers a relatively small frequency range proves to be disadvantageous. Alternatively, integration of the radio antenna in the surveyor's pole is also known, but the potential uses of the surveying station are limited thereby to the application on the surveyor's pole. On the other hand, potential uses of the GNSS surveying station on a total station, a tripod or a support would also be desirable.
A further GNSS receiver station known in the prior art is realized with a rod antenna which is arranged outside the housing, pointing upwards and centrally on the GNSS station. However, it is not possible to rule out the fact that the GNSS signals received by the GNSS antenna located underneath and hence the GNSS accuracy will be influenced thereby. Furthermore, during use on the ground, this solution is also susceptible to damage and awkward since care is always required.
U.S. Pat. No. 6,751,467 and U.S. Pat. No. 7,110,762 describe, as prior art, a GPS receiver in a housing in which a radio modem receiver, below the GPS receiver, and a radio antenna are additionally integrated. The radio antenna geometry in the form of a slot antenna is mounted on a polyimide sheet. The sheet with the antenna geometry is rolled into a cylinder and arranged below the GPS antenna, around the radio modem receiver, in the housing of the GPS receiver. However, the radiation characteristic of the antenna arrangement described has a strong directional dependency; furthermore, this antenna arrangement covers a relatively small frequency range and has a large space requirement.
U.S. Pat. No. 5,831,577 and U.S. Pat. No. 5,691,726 disclose an antenna combination comprising a GNSS antenna which has an upward-pointing, radiating circuit board and a motherboard underneath, and a wire loop antenna which is arranged as a winding below or above the GNSS antenna and is formed for receiving radio signals with correction information. In the case of the arrangement of the wire loop radio antenna above the GNSS antenna, however, it is not possible to rule out an influence on the GNSS signals received by the GNSS antenna underneath and hence on the GNSS accuracy. In the case of the arrangement below the GNSS antenna, the radiation characteristic of the wound radio antenna is influenced by the GNSS antenna. Also disadvantageous is the poor compatibility with further systems, since only a small frequency range is covered by the wire loop radio antenna and it is therefore possible to communicate via radio only with devices tuned thereto. Thus, it is usual for some providers to have to equip their GNSS receiver units for different areas of use (Europe, USA) and for use in different systems, each with antenna geometries specialized appropriately therefor.
In summary, negative mutual influences on the antennas arranged in the housing occur in the case of solutions to date for GNSS receivers; further solutions are susceptible to destruction and are not easy to handle during use on the ground (for example breakage of an externally arranged antenna), are not compact or have poor compatibility for communication of correction data with further units.